The dissolution experiment of anorthite was carried out under different partial pressures of CO₂ (P_CO2) to understand the role of weathering of Ca and Mg silicates in the removal of CO₂ from the atmosphere in the Precambrian. A newly-developed apparatus can control P_CO2 in a mineral-water reaction vessel and vary P_CO2 and temperatures from 10^−3.5 (the present atmospheric level, PAL) to 5.0 atm and from 40 to 150°C, respectively. The experiments were carried out at 150°C and pH 4.56, varying P_CO2 from 10^−3.5 to 1.2 atm. The dissolution rate of anorthite under high P_CO2 (=1.2 atm) is faster than that under PAL by about 4 times. Calcium and Si dissolve congruently from anorthite under PAL, whereas a Ca/Si ratio in solution is lower under high P_CO2. Our results suggest that the weathering rates and processes in the Archean are different from the present ones, and that the amount of Ca flux into the Archean ocean is less than that predicted assuming congruent dissolution of anorthite.

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Hexahydroborite was found in a vein consisting of borate minerals which developed along the boundary between crystalline limestone and skarns at Fuka, Okayama Prefecture, Japan. Hexahydroborite occurred as aggregates of pyramidal crystals up to 2mm wide on the cavity wall, in association with olshanskyite and calcite. Wet analyses and ICP-MS gave the empirical formula Ca_1.001B_2.102O_4.154·5.846H₂O on the basis of O=10, which was consistent with the ideal formula Ca[B(OH)₄]₂·2H₂O. The reflections of X-ray powder data for hexahydroborite from Fuka were indexed on the monoclinic cell, a=16.011(2), b=6.688(1), c=7.954(2)Å, β=103.81(1)°, determined by single crystal method. The mineral was optically biaxial positive with refractive indices α=1.502(2), β=1.505(2), γ=1.509(2) and had a Mohs hardness of 2.5 and a density of 1.84gcm⁻³. It is likely that hexahydroborite at Fuka was formed by a reaction of ground water with sibirskite and/or parasibirskite at a low temperature around 25°C.

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We have examined synthetic single crystals of forsterites shocked up to 82GPa using a single stage propellant gun. Even in forsterites shocked to 82GPa, any decomposition phases such as spinels or silica glasses and fine-grained MgO could not be detected by X-ray diffraction and EPMA methods. Profile analysis of X-ray diffraction peaks of the shocked forsterites using a Gandolfi camera has been done to detect the maximum strain. The maximum strain derived from Williamson-Hall plot of about 20 diffraction angles and their integral maximum breadths had a linear relation to the shock pressures, in which the elastic limit could not be detected. Unit cell dimensions did not show any changes against the shock pressures, however the increasing broadening of X-ray diffraction peaks occurred with the increase of shock pressures, indicating the homogeneous lattice distortion due to shock pressures. This is the first quantitative analysis that enables us to estimate an experienced shock pressure from the X-ray peak broadening, and is applied to the Dhurmsala LL6 chondrite; the pressure estimated in this study is ranging from 24.0 to 37.8GPa in contrast to the range of 5 to 20 GPa (shock stage S3 by Stöffler et al., 1991) estimated from the texture of olivine in the chondrite.

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